Fuel cell systems are increasingly being used as a power source in a wide variety of applications. Fuel cell systems have been proposed for use in power consumers such as vehicles as a replacement for internal combustion engines, for example. Such a system is disclosed in commonly owned U.S. patent application Ser. No. 10/418,536, hereby incorporated herein by reference in its entirety. Fuel cells may also be used as stationary electric power plants in buildings and residences, as portable power in video cameras, computers, and the like. Typically, the fuel cells generate electricity used to charge batteries or to provide power for an electric motor.
Fuel cells are electrochemical devices which combine a fuel such as hydrogen and an oxidant such as oxygen to produce electricity. The oxygen is typically supplied by an air stream. The hydrogen and oxygen combine to result in the formation of water. Other fuels can be used such as natural gas, methanol, gasoline, and coal-derived synthetic fuels, for example.
The basic process employed by a fuel cell is efficient, substantially pollution-free, quiet, free from moving parts (other than an air compressor, cooling fans, pumps and actuators), and may be constructed to leave only heat and water as by-products. The term “fuel cell” is typically used to refer to either an individual cell or a plurality of cells depending upon the context in which it is used. The plurality of cells is typically bundled together and arranged to form a stack with the plurality of cells commonly arranged in electrical series. Since individual fuel cells can be assembled into stacks of varying lengths, systems can be designed to produce a desired energy output level providing flexibility of design for different applications.
The stacks may comprise of more than one hundred individual bipolar plates, wherein successive plates and a membrane-electrode-assembly (MEA) disposed therebetween form the individual cell. Typically, apertures formed in successive bipolar plates cooperate to form a “header” running the length of the fuel cell stack. The plate formed header distributes reactants (such as oxygen and hydrogen) and coolant to the individual cells. A first end of the plate formed header is sealingly disposed against an end unit, wherein an injector, a recycler, a reactant source, a humidifier, or other support system is typically disposed. A second end of the plate formed header is sealingly disposed against an end plate or a second end unit.
Bipolar plates include active and inactive areas formed thereon. The electrochemical reaction occurs in active areas of the bipolar plates. Inactive areas are used to guide reactants and coolants across portions of the plate, provide sealing surfaces for gasket material, form apertures in the plate, and provide structural support for the plate. Large areas of inactive areas on plates result in an inefficient use of the plate and a gasket used to form the individual cells.
Fuel cell stacks of varying number of cells require different amounts of reactants and coolant to operate properly. Apertures formed in the bipolar plates may be sized to optimize reactant and coolant flow rates to and from the fuel cell stack. A stack having a larger number of cells, and thus a longer stack length requires plate formed headers capable of carrying more reactants and coolant, necessitating larger apertures in the plates. As a result, a particular plate design is limited to a relatively narrow range of stack lengths, and a manufacturer may be required to support multiple plate designs to accommodate a number of vehicles having a large differential of energy requirements.
Fuel cell stacks require a close stacking alignment and adequate sealing between successive plates. Sealing surfaces formed on the plates and the MEAs must be properly aligned to form the fuel cell stack that operates efficiently, militates against leakage of reactants and coolant, and electrically isolates successive plates from one another.
Plate formed headers have a consistent cross-sectional shape along a header length when a single plate design is used to form the fuel cell stack. A consistent cross sectional shape may be undesirable for to the fuel cell stack because a pressure differential may exist along the length of the header, causing differences in reactant and coolant flow rates into individual cells. Additionally, plate formed headers limit header access to ends of the fuel cell stack for placement of components such as a distribution manifold, a water separator, and an infector, for example, necessary for operation of the fuel cell stack.
It would be desirable to produce a discrete header for a fuel cell stack, wherein the discrete header minimizes use of the plate and the gasket materials, allows a single plate design to be used for multiple stack lengths having a large differential of energy requirements, provides a durable alignment mechanism for the fuel cell stack, and provides integration flexibility for components and configurations of the fuel cell stack.